You don't. Windows does. What OS is it? And 32bit or 64bit?
If you have a 32bits OS and installed more than 3GB RAM, you need to upgrade to a 64bits OS - assuming you have a 64bits CPU.
If you already have a 64bits CPU, 64bits OS, check that the BIOS is recognizing the extra RAM. If the BIOS doesn't see it then you either exceeded your platform's maximum RAM capacity or have a hardware compatibility issue.
Lots of ifs since you didn't say anything about your CPU/RAM/MoBo.
If you'd like more help, please state:
1. How much RAM you installed (4GB, 8GB?)
2. Do you have 64-bit or 32-bit Windows?
3. If 32-bit Windows, how much VIDEO RAM does your graphics card/chip have?